jQuery Grid Plugin - jqGrid

I've found that this works well for custom searching/filtering:

     $.extend(postdata,{filters:'',searchField: 'ColumnName', searchOper: 'eq', searchString: 'test search string'});

But what if I want to filter records that don't have an empty string in a particular column?  Why doesn't this work?

     $.extend(postdata,{filters:'',searchField: 'ColumnName', searchOper: 'ne', searchString: ''}); 

     OR

     $.extend(postdata,{filters:'',searchField: 'ColumnName', searchOper: 'ne', searchString: null});